1. Field of the Invention
The invention relates to a method for the production of a multilayer pipe of thermoplastic material, in particular polyolefine, and an installation for putting the method into practice.
2. Background Art
Multilayer pipes, in particular three-layer pipes, are known from U.S. Pat. No. 3,538,209 and U.S. Pat. No. 3,677,676.
German published patent application 20 42 031 teaches a method and an apparatus for the production of three-layer pipes of thermoplastic material, a twin-wall pipe provided with an exterior corrugated pipe being produced in usual manner. A tube is injection-molded around this twin-wall pipe in a coextruding head, the tube being vacuum-calibrated in a downstream external calibrating arrangement. Three-layer pipes produced in this way can be used as protective pipes, in particular as cable ducts and drain channels, if they are not exposed to any substantial pressure from the inside. The problem with this way of producing multi-layer pipes resides in that no reliable welding is obtained between this tube for the production of an exterior pipe or a skin and the pipe produced and externally shaped in advance. In particular when polyolefines, such as polyethylene, are used, the pipe produced and externally shaped in advance can exhibit considerable tolerances in outside diameter, there being no defined conditions for the known external vacuum calibration.